1. Field of the Invention
This invention relates generally to an apparatus for controlling a switching type amplifier and more particularly, to an apparatus for controlling such a switching amplifier that can be used in power line communication systems such that, during fault conditions, the output current at the drive portion of the switching amplifier is limited.
2. Description of the Prior Art
The practice of utilizing a switching type power amplifier to provide an efficient power gain when interfacing digital circuits to linear requirements has proven effective for a wide range of applications. One such application is in the area of power line communication systems where an electric utility company desires to perform a load management operation from a central location, an example of such a load management operation being a load shedding command which the utility would use to shed a non-essential current consuming load from the power line during peak demand periods. To perform this operation, the utilities have typically employed techniques for coding the necessary commands and transmitting the information over the power line. In communicating such information over the power line, the communication technique of phase shift keying (PSK) has been utilized, one application which is representative of such technique is found in U.S. Pat. No. 3,911,415 issued on Oct. 7, 1975 to Whyte. In this patent, a baseband data message is used to modulate a carrier signal which typically has a frequency in the audio frequency range and for this example is chosen as 12.5 kilohertz, although it can be appreciated that other frequencies and coding techniques can be used as well. The resultant modulated carrier signal, which experiences a phase shift when the leading or trailing edge of the baseband signal occurs, is injected onto the 60 hertz current flowing in the power line. At a remote location, which can typically consist of a load management terminal for which the command is intended to be operative, a receiver demodulates the carrier signal. U.S. Pat. No. 4,355,303 which issued on Oct. 10, 1982 to Phillips et al. is representative of the type of receiver used at the remote locations of the power line communication system.
When the communication system must accommodate a large geographical area of operation, it has been necessary to utilize a signal repeater for receiving the transmitted signal and retransmitting such signal at a sufficient power level to be received at the most distant location. U.S. Pat. No. 3,962,547 which issued on June 8, 1976 to Pattantyus discloses such a signal repeater device.
At the central location where the load monitoring and shedding commands originate and the associated controls are assembled for transmission over the power line, the message is first used to modulate a carrier signal using the PSK method as previously discussed. Before being injected onto the power line, however, the squarewave signal must first be converted to an analog signal peculiar to the requirements of the power line. One technique for accomplishing this is by way of a switching type power amplifier which utilizes metal oxide semiconductor field effect transistors (MOSFET). The MOSFET transistor is utilized primarily because of its property for fast switching responses to changes in the input voltage waveform thereby providing an output signal which is synchronized with the phase and frequency of the input signal. In applying such technology to a power line communication system, it is necessary to consider the effects of short circuit and/or low impedance conditions at the output stage of the MOSFET transistor. The condition of the power line, whether of a normal conducting condition or of a fault type condition, is determinative of the magnitude of the switching amplifier current output. If a fault condition arises on the power line, the magnitude of the output current signal can fluctuate to the extent that a potentially destructive condition can occur at the output stage of the MOSFET transistor. In fact, if a sustained short circuit condition would exist, the switching transistors would over saturate causing excessive heating such that component failure would eventually result.
One approach to limiting the current at the output of the switching amplifier has been to introduce a burden having a preselectable impedance; this approach however suffers in that a significant amount of power is dissipated which also adds to the overheating problem. Additionally, to accomplish this approach, additional components are needed which unduly increases the cost of the switching amplifier.
Another approach to limiting the current at the output stage of the switching amplifier can be found in U.S. Pat. No. 4,645,956 which issued on Feb. 24, 1987 to Shuey. In this patent, the problem of overheating of the output transistors during fault conditions associated with the power line is recognized with the solution advanced therein involving detecting the occurrence of an overcurrent condition and reversing the drive of the output transistors at this time. For short durations of the short circuit or low impedance condition, this approach is effective for limiting the current flow through the switching transistors, however, this advantage has associated therewith, some increased harmonic content at the output waveform.